1. Technical Field
The present invention relates to a method of manufacturing a semiconductor device, a semiconductor device, and a semiconductor apparatus, and more particularly, to a method of manufacturing a semiconductor device including an electrode making ohmic contact with a silicon carbide bulk substrate, a semiconductor device, and a semiconductor apparatus.
2. Description of Related Art
In the related art, semiconductors made of silicon carbide (hereinafter, also referred to as SiC) are very suitably used in a semiconductor substrate because they yield excellent physical properties having a wide band gap characteristic, and the constituent elements thereof are nearly inexhaustibly present in the natural world.
Since the SiC has a covalent bond crystal structure, it is very stable in a physical sense, and has a high band gap and a high melting point. For this reason, in a case where the ohmic electrode is formed in the silicon carbide (SiC) semiconductor substrate, it is necessary to perform heat treatment (such as annealing) at a higher temperature than a temperature used to form a Schottky electrode.
As a heat treatment method for obtaining ohmic contact between the semiconductor substrate and the electrode, there has been proposed a method of irradiating laser light with an irradiation energy level capable of preventing crystal materials from becoming molten to an ion-implanted SiC semiconductor substrate (for example, refer to Patent Documents 1 and 2).
In addition, there has been proposed another method in which a reflectance control film, by which reflectance is reduced as the film thickness is reduced for irradiation of laser light, is formed on a semiconductor substrate, the reflectance control film is etched, and then, the laser light is irradiated to the semiconductor substrate for performing an annealing process (for example, refer to Patent Document 3).
However, in a case where heat treatment is performed for forming an ohmic electrode after forming a Schottky electrode in the SiC semiconductor substrate, characteristics of the Schottky electrode may be degraded due to a high temperature. Therefore, in the related art, it is necessary to form the ohmic electrode before forming the Schottky electrode in order to manufacture the semiconductor device using the SiC semiconductor substrate.
Hereinafter, a process of forming the Schottky electrode in the SiC semiconductor substrate will be exemplarily described for top-and-bottom electrode type Schottky barrier diodes 100 and 200 shown in FIGS. 6A and 6B.
As shown in FIG. 6A, a series resistance within a device structure of the Schottky barrier diode 100 can be expressed as a sum of a resistance at an interface between the Schottky electrode 122 and the SiC semiconductor epitaxial layer 121 and internal resistances of the Schottky electrode 122, the SiC semiconductor epitaxial layer 121, the n+ SiC semiconductor substrate 110, and the ohmic electrode 130. A lower series resistance yields reduced operational power loss and excellent device characteristics. For this reason, in order to effectively reduce the series resistance of the Schottky barrier diode, there has been proposed a method in which the Schottky barrier diode may be thinned by polishing the n+ SiC semiconductor substrate 210 while the Schottky electrode 222, the SiC semiconductor epitaxial layer 221, and the ohmic electrode 230 are left as is to maintain other characteristics as in the Schottky barrier diode 200 shown in FIG. 6B. However, if the n+ SiC semiconductor substrate 210 is thinned too much, the substrate strength is reduced so that the semiconductor device may be broken.
For this reason, in the related art, the semiconductor device having the Schottky electrode formed on the SiC semiconductor substrate has been used without polishing the SiC semiconductor substrate. However, in a case where the SiC semiconductor substrate is used without the polishing, the thickness of the substrate inevitably increases, and the series resistance within the device structure also increases as described above, which may cause a lot of power loss and degradation of device characteristics.
Here, in order to obtain ohmic contact between the semiconductor substrate and the electrode without performing the heat treatment described above, there has been proposed a method in which the semiconductor device structure is formed in the main surface side of the silicon semiconductor substrate, the rear surface side of the semiconductor substrate is polished in a final process, impurity ions having the same conduction type as that of the rear surface are implanted, and then, a metal thin film corresponding to the ohmic electrode is formed (for example, refer to Patent Document 4). However, in a case where the top-and-bottom electrode type semiconductor device is manufactured using the method disclosed in Patent Document 4, it is not possible to form the ohmic electrode before forming the Schottky electrode. For this reason, as described above, characteristics of the Schottky electrode may be degraded due to a high temperature during the heat treatment of the ohmic electrode.
In addition, there has been proposed a method including a process of depositing a metal film on the SiC semiconductor substrate and a process of performing heat treatment on a metal film by irradiating laser light from the rear surface side of the SiC semiconductor substrate to form an ohmic electrode by making ohmic contact between the metal film and the SiC semiconductor substrate (for example, refer to Patent Document 5). According to the method disclosed in Patent Document 5, it is possible to selectively perform heat treatment only on a portion where the ohmic electrode is formed by irradiating laser light from the rear surface side of the SiC semiconductor substrate. Therefore, it is possible to suppress the SiC semiconductor substrate and the device structure from being damaged.
However, for example, in a case where the manufacturing method disclosed in Patent Document 5 is applied to the top-and-bottom electrode type semiconductor device as shown in FIGS. 6A and 6B, a high-temperature heat is easily transferred from the rear surface side to the main surface side along the thickness direction of the SiC semiconductor substrate. For this reason, characteristics of the semiconductor device structure including the Schottky electrode or the semiconductor layer may be degraded.